In recent years, there has been proposed a technology for directly performing communication of user data (data of a User-Plane) among a plurality of radio terminals without going through a radio base station. This technology is also called D2D (Device to Device) communication. The user data is communicated directly among the plurality of radio terminals by using a part of radio resources assigned to a mobile communication system. However, in the D2D communication, data of control data (C-Plane) is communicated via the radio base station, similarly to a conventional mobile communication system.